NVIC does not tail-chain to PendSV ISR

Question

I'm trying to implement context switching from handler mode to thread mode using PendSV. I set the PendSV pending bit inside an ISR with SCB->ICSR = SCB->ICSR | SCB_ICSR_PENDSVSET_Msk . After setting the PendSV pending bit, I can see that the PendSV exception is pending: SCB->ICSR = 0x10000813 (I think the ISRPREEMPT bit should be set as well but it's not). 
 NVIC should tail-chain to my PendSV handler before unstacking. Instead ISR unstacking occurs, a return to exception is executed, and a few thread-level instructions are executed, before the PendSV handler is called. 
 The SoftDevice is present but has not been enabled. 
 Any ideas on what might be causing this? Thanks.

Susheel Nuguru · Accepted Answer

This is how i verified it. 
 void PendSV_Handler(void)
 {
 volatile register uint32_t r0 __asm("r0");
 volatile register uint32_t r1 __asm("r1");
 printf("PendSV %x
",r0);
 printf("PendSV %x
",r1);
 }

void TIMER1_IRQHandler(void)
{
 volatile register uint32_t r0 __asm("r0");
 volatile register uint32_t r1 __asm("r1");
 
 printf("Timer1 %x
",r0);
 printf("Timer1 %x
",r1);
 
 
 NRF_TIMER1->INTENSET = 0;
 NVIC_DisableIRQ(TIMER1_IRQn);
 NVIC_ClearPendingIRQ(TIMER1_IRQn);

 SCB->ICSR = SCB->ICSR | SCB_ICSR_PENDSVSET_Msk;

 int i = 0xFFFF;
 while(i--); 

 __asm("mov r0, #0xbeef");
 __asm("mov r1, #0xdead");
}



/**
 * @brief Function for main application entry.
 */
int main(void)
{
 uint32_t err_code;

 simple_uart_config(0, TX_PIN_NUMBER, 0, RTS_PIN_NUMBER, false); // Hardware flow control not used in this example.
 
 printf("
Start: 
");

 NVIC_SetPriority(TIMER1_IRQn, 2);
 NVIC_EnableIRQ(TIMER1_IRQn);
 NVIC_SetPriority(PendSV_IRQn, 0xFF);
 NVIC_EnableIRQ(PendSV_IRQn);
 NVIC_SetPendingIRQ(TIMER1_IRQn);
 
 while(1);
}
 
 I set 0xdeadbeef in R0 and R1 in the end of Timer1 ISR and i see that they retain on entry to PendSV, this can only happen with tail chaining 
 OUTPUT 
 Start:
Timer1 0
Timer1 e000e200
PendSV beef
PendSV dead

NVIC does not tail-chain to PendSV ISR

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